I've seen this problem a long time now.

When you have enabled the "Popup logout window", wich you have since it
cannot be disabled curently :-), the Monowall is sending something more then
just an ordinary HTTP Get request to the original requested page.
I also get a blank page if I enable the logout windows, but if I do a
refresh on that blank window the requested page will show the way it should.

Don't know exactly what type of info it is sending, but I guess it is
posting info about requested URL or such.
Maybe someone in the Monowall Team can help us with this issue...

/ M